The invention relates generally to radiation tolerant electronic circuits and in particular to a transient ionizing radiation tolerant peripheral system for use with a magnetic bubble memory.
As semiconductor based electronic systems have become more complex and faster, they have also become more vulnerable to perturbation and upset by external phenomena including ionizing radiation. In particular, transient ionizing radiation of the type produced by fission and fusion weapons can profoundly disturb semiconductor systems, including microcomputer based systems used for command and control in national defense. A typical transient ionizing radiation event is of relatively short duration, lasting less than 50 nanoseconds; however, the effects of the radiation on semiconductors may last for several microseconds.
Transient ionizing radiation interacts with semiconductor material, in particular with doped silicon, by producing extra electron and hole pairs within the semiconductor crystal. The surplus electrons and holes cause a temporary increase in the number of charge carriers in the semiconductor. The charge carriers are those charged bodies which are free to move in response to an electric field of a few volts, a field strength typically applied to semiconductor integrated circuits. The unwanted radiation induced increase in conductivity and in current amplitudes can cause logic or memory devices to change state and can even lead to damage from transient currents which overheat and burn out portions of the integrated circuits. The increase in available charge carriers also can increase the electric field across the gate oxides of field effect transistors, leading to punch-through of the oxides or latch-up of CMOS integrated circuits due to abrupt increases in potentials within the circuits. On a temporary basis, the increase in the number of charge carriers can also cause changes in the states of semiconductor memories leading to corruption of stored data which can cause catastrophic system failure. In addition, spurious signals may be circulated throughout such a system and cause system failure.
One method for avoiding the deleterious effects of the transient ionizing radiation is to employ memories which are relatively impervious to the radiation. One such memory is a magnetic domain bubble memory or magnetic bubble memory. Magnetic bubble memories consist of a substrate having a thin, epitaxial magnetic layer formed thereon. The epitaxial magnetic layer has its crystalline structure oriented so that it may be magnetized transversely to the plane of the epitaxial layer, as opposed to parallel to the plane of the layer as for most common magnetic materials. The epitaxial magnetic layer is orthoferrite or in most cases, doped garnet. Normally, the magnetic domains in doped garnet organize themselves into serpentine or sinuous patterns. If a static magnetic bias field normal to the epitaxial film layer is applied, the serpentine patterns tend to shrink until right circular cylindrical magnetic domains are formed which are oriented parallel to the applied static bias field. These cylindrical magnetic domains are also known as magnetic bubble domains or magnetic bubbles.
The magnetic bubble memory epitaxial layer has a plurality of chevrons formed thereon. The chevrons consist of soft magnetic material and are used to propagate the magnetic bubbles in preferred directions in the epitaxial layer. The magnetic bubbles can be moved by application of an in-plane magnetic field gradient. Typically, the field is changing or rotating and is generated by currents flowing in a pair of coils, one coil producing a magnetic field having a cyclically varying magnitude and oriented in an X direction, the other coil producing a magnetic field having a cyclically varying magnitude and oriented transversely to the X direction in a Y direction. Both the X and Y directions are aligned with the plane of the epitaxial magnetic layer. Typically, the coils are each energized by a respective rectangular wave voltage. The voltages are separated from one another by 90.degree., thereby causing the resulting field from the coils to rotate 360.degree. during each period of applied voltage. Each 360.degree. rotation of the magnetic field causes each of the magnetic bubbles in the epitaxial layer to move from a selected or parked position on one chevron to an identical selected or parked position on an adjacent chevron.
The circuits which generate the rotating field currents are typically composed of semiconductor material and may fail temporarily when exposed to transient ionizing radiation. A typical failure of a rotating field coil driver causes the current in the rotating field coil to remain constant for a short period of time thereby becoming delayed or out of phase with the signals from other portions of the system. As a result, the magnetic bubbles will not have reached their proper parking positions at the end of the next timing cycle, causing the information stored in the magnetic bubble memory to be lost.
As mentioned above, the magnetic bubble memories operate by circulating magnetic bubble domains throughout an epitaxial magnetic layer or film. Binary information is stored in the magnetic epitaxial layer in the form of the presence or absence of magnetic bubbles. The magnetic bubbles are formed by nucleating a magnetic bubble or by replicating which entails cutting and transferring a magnetic bubble. A swap operation directs the newly formed magnetic bubble to a storage loop where it is circulated by the rotating magnetic field. In an output operation, data are transferred out of the storage loops by means of a replicate operation.
The replicate, generate and swap operations require relatively large amounts of current to be supplied to replicate, generate and swap gates on the epitaxial layer which produce magnetic fields in order to cut, nucleate, transfer or swap the magnetic bubble domains. In the event that a conventional bubble memory is exposed to transient ionizing radiation, the radiation can either create current surges or cause a loss of current in the semiconductor circuits which operate the function gates. This leads to the creation of spurious data which are then stored within the bubble memory.
Bubble memories also have output cycles wherein bubbles circulating in storage loops in the epitaxial layer are replicated to form bubbles in an output loop. The bubbles in the output loop are propagated by the rotating field to a magneto-resistive sensor which drives a sense amplifier to produce an output signal in response to the presence or absence of a bubble. The swapping operation also requires electrical current to be supplied to a swap gate which is a magnetic field generating device. The swap current may be disturbed by transient ionizing radiation perturbing the semiconductor circuits which generate it. As a result, data which have been correctly stored may be destroyed on output due to transient ionizing radiation influencing the swap circuits.
In order to avoid problems with transient ionizing radiation those skilled in the art previously employed a backup magnetic bubble memory as well as the on-line memory. The backup bubble memory contained all of the information stored in the on-line memory. The system was also equipped with a radiation sensor which reset the system and switched over to the backup memory when a transient ionizing radiation pulse was received. The problem with such a system is that it is relatively expensive because it employs wholly redundant bubble memories, which might be used for only a fraction of a second during the life of the system. In addition, if the prior system received a number of transient ionizing radiation pulses in rapid succession, the system would quickly run out of good copies of information even if multiple backup bubble memories were used.
It is also well known to protect circuits exposed to high potentials, such as lightning strikes, by providing current shunting paths to carry away extra current. Lightning arrestors perform this function by employing spark gap or arcing devices which become conductive at high potentials to carry potentially damaging current surges around the device to be protected and to ground.
What is needed is a system having semiconductive circuits which can tolerate levels of transient ionizing radiation that do not exceed radiation levels tolerated by central processing, logic or timing circuits.